Several methods for depositing thin films on substrates (i.e., deposition targets) are known in the art. Such methods include physical vapor deposition and chemical vapor deposition. One common form of thin film deposition is evaporation deposition. Evaporation deposition involves vaporizing a deposition or source material under high vacuum, and condensing the vaporized deposition material on a substrate to form a thin film. Operating under vacuum conditions allows the vaporized deposition material to travel unhindered in a direct path to the substrate where it condenses back to a solid state. This process is used to produce thin films with a relatively high level of purity desirable in many applications. The deposition material can be selected from any vaporizable material including, but not limited to, pure elements, compounds, metals, alloys, ceramics, oxides, semiconductors and mixtures thereof.
The typical evaporation deposition device includes a deposition chamber, a vacuum pump for evacuating the deposition chamber, a movable substrate carrier for transferring and supporting a substrate into the deposition chamber, and one or more fixed evaporation or point sources usually located opposite from the substrate. Typically, evaporation deposition is a continuous production process where the substrate is moved through a series of deposition chambers for depositing one or more layers of same or different materials thereon. The substrate carrier can be configured for transferring or moving the substrate from one deposition chamber to another during production.
During the film deposition process, the point sources vaporize the deposition material and direct the resulting vapor flux to the substrate. The point source typically includes a crucible for holding the deposition material, a vaporizer for supplying the energy, typically thermal energy, needed for vaporizing the deposition material (e.g., a heat generator), and an opening in the crucible through which the vaporized deposition material is directed to the substrate in the deposition chamber.
Evaporation deposition is used in the fabrication of semiconductor devices such as, for example, thin film solar cells and modules. Thin film solar cells or modules employ, inter alia, thin films of absorber materials composed of elements, binary compounds, ternary compounds and multinary compounds selected from Group I, Group II, Group III, Group IV, and Group VI of the periodic table. One type of absorber material is composed of combinations of copper, indium, gallium, selenium and sulfur (CIGS(S)). The thin films of absorber material are typically deposited on a substrate coated with a back contact metal layer. It is very important that these thin films be deposited with a high degree of uniformity to realize the electrical and mechanical properties necessary for the production of a useful solar cell.
Referring to FIG. 1, a prior art evaporation deposition device 10 includes a housing 12 defining a deposition chamber 14, a movable substrate carrier assembly 16 transporting and supporting a substrate 18 in position at the upper end of the chamber 14 in an operable position for receiving the deposition material, and one or more fixed point sources 20 (two fixed point sources are shown) located at the end of the chamber 14 opposite the substrate 18. The substrate carrier assembly 16 includes a pair of opposed slidably movable railings 17 each supporting opposite ends of the substrate 18, and configured to move the substrate 18 into and out of the deposition chamber 14. The substrate carrier assembly 16 can be adapted for conveying or transporting the substrate 18 between two or more deposition chambers 14 for continuous processing or production. The substrate carrier assembly 16 may be selected from any other suitable substrate transport mechanism as known in the art (e.g., conveyor assembly).
The fixed point sources 20 are maintained in a spaced apart arrangement within the deposition chamber 14 and remain in a fixed position such that the longitudinal axis passing through the point source 20 is contiguous with the longitudinal axis of the opening leading to the deposition chamber 14. The term “contiguous” as used herein means that the line passing through the center of the opening leading to the chamber 14 will likewise pass through the center of the fixed point source 20.
Each point source 20 includes a crucible 22, containing a deposition material 24 and a vaporizer or source of energy (not shown) operatively associated with the crucible 22 for vaporizing the deposition material 24. An example of such sources of energy is an electrical heating device for supplying sufficient heat to vaporize the deposition material 24. The crucible 22 further includes a fixed opening 26 having a longitudinal axis 29 through which the vaporized deposition material is directed to the substrate 18 as a vapor flux plume 28. The longitudinal axis 29 is likewise the longitudinal axis passing through the crucible 22. The plumes 28 produced by the point sources 20 are directed to the substrate 18 in a manner such that a portion of opposed plumes 28 overlap one another forming an overlap region 32.
FIG. 1 shows a normal distribution curve 30 which represents the relative concentration of vaporized deposition material from the center to the lateral sides of the vapor flux plume 28 for a given sized opening 26. The normal distribution curve 30 of fixed point sources 20 demonstrates a relatively narrow bell curve indicating the concentration of the vaporized deposition material is highest at the center of the plume 28 (generally corresponding to the longitudinal axis 29 of the opening 26) with the concentration of the vaporized deposition material 24 diminishing sharply toward the lateral sides of the plume 28. The plumes 28 are directed by the point sources 20 with a portion of each overlapping one another in the overlap region 32.
Referring to FIG. 2, there is shown a top view of the substrate 18 having a thin film 34 deposited by the fixed point sources 20 thereon produced by the type of evaporation deposition device 10 shown in FIG. 1. The deposition pattern of the thin film 34 includes non-uniform deposition areas. More specifically, the thin film 34 includes two areas 36 of relatively dense deposition, a central area 38 of relatively intermediate density deposition and two side areas 40 of relatively low density deposition. The high deposition areas 36 of the thin film 34 are produced by exposure of the substrate 18 to the center portion of the respective plumes 36. The intermediate deposition area 38 of the thin film 34 is produced by exposure of the substrate 18 to the overlap region 32 of the lateral sides of the plume 28. The low deposition areas 40 of the thin film 34 are produced by exposure of the substrate 18 to the lateral sides of the respective plumes 28. (i.e., the areas remote from the center portion of the plumes 28.)
Referring again to FIGS. 1 and 2, the size of the opening 26 can have an effect on the deposition pattern. For example, if the size of the opening 26 is increased (i.e., increased area), the width of the distribution curve 30 increases and the steepness of the center portion of the bell curve is reduced (i.e., becomes less pronounced). This adjustment tends to spread the deposition material over a greater area of the substrate 18 but also increases that part of the distribution curve 30 that does not contact the substrate 18. Overall, the substrate 18 characteristically exhibits an uneven pattern of deposition material.
Similarly, if the size of the opening 26 decreases, the substrate 18 will exhibit a different, but still uneven pattern of distribution. In particular, as the width of the distribution curve decreases, the steepness of the center portion increases. Thus, the distribution of deposition material is focused on a smaller area of the substrate 18 and the overlap region 32 will be reduced or eliminated.
It can readily be seen that varying the size of the opening 26, while varying the pattern of distribution does not achieve a desired uniform deposition over the entire substrate 18. In order to obtain uniform deposition under these prior art conditions, it is often necessary to subject the substrate to post-deposition operations to alter the deposition pattern on the substrate 18. Post deposition operations add significantly to the cost and time of producing finished substrates.
The use of fixed point sources 20 significantly limits the ability of thin film deposition devices to produce thin films 34 with a high degree of uniformity or desired thickness patterns, or to readily accommodate substrates of different sizes and/or shapes. This is due in part to the fixed position of the longitudinal axis of the opening leading to the chamber and the point source. In particular, the longitudinal axis of the chamber and the point source are in a fixed orientation and do not vary.
One approach to overcome the uneven deposition patterns is to periodically interrupt the deposition process (i.e., shutting off or taking the evaporation deposition device offline) to perform adjustments needed to achieve the desired film uniformity, thickness pattern and the like. This approach is very costly, time consuming, and labor-intensive.
In view of the foregoing problems and limitations, there is a need for a point source assembly and thin film deposition devices employing the same for depositing thin films, that enables adjustment of deposition parameters in situ to produce thin films on a substrate with a high degree of uniformity or desired thickness patterns in a cost effective manner. There is a further need for a point source assembly and thin film deposition devices employing the same for depositing thin films, that enhances production yield, reduces downtime, and/or reduces the number of point sources needed to achieve desired deposition uniformity.